The present invention relates to an actuator for a fuel injector, and more particularly to a fuel injector actuator having a plurality of sets of piezoelectric elements.
A conventional piezoelectric element is a ceramic structure whose axial length changes in the presence of an electric field created by applying a voltage across the element. In typical applications, the axial length of the element can change by, for example, approximately 0.12%. In a stacked configuration of elements, the change in the total axial length of the stack is equal to the sum of the changes in axial length of each element in the stack. As is known, applying a voltage to a piezoelectric element, or to a stack of piezoelectric elements, results in a nearly instantaneous expansion of the actuator and an instantaneous movement of any structure connected to the actuator.
It is known to use a single set of piezoelectric elements, i.e., a stack of piezoelectric elements across which a common voltage is applied, to actuate a fuel injector for an internal combustion engine. Such piezoelectric actuators precisely open and close an injector valve element for precisely metering fuel flow into a combustion chamber.
The thermal and pressure effects present in the piezoelectrically actuated injector""s operating environment can cause dimensional changes within the injector. These dimensional changes result in a change to the injector""s stroke, causing an unstable shift in its flow characteristics. To compensate for the dimensional changes, it is known to fabricate injectors from exotic materials, which exhibit low thermal expansion. In addition, it is also known to calibrate injector strokes to anticipate elongation of the valve body. However, these methods are costly and inefficient.
Advantages of the claimed invention include increasing the stroke of the piezoelectric stack, compensating for thermal expansion in different operating condition, and compensating for mechanical deformation under different fuel pressures and assembly stresses.
The present invention provides a fuel injector that comprises a tube assembly having a longitudinal axis extending between a first end and a second end; a seat secured at the second end of the tube assembly and defining an opening; a stem assembly movable with respect to the seat, the stem assembly moving between a first position wherein the stem assembly contiguously engages the seat such that fuel flow through the opening is prevented and a second position wherein the stem assembly is spaced from the seat such that fuel flow through the opening is permitted; a first set of piezoelectric elements moving the stem assembly in response to a first electric field; and a second set of piezoelectric elements moving the first set of piezoelectric elements in response to a second electric field.
The present invention also provides a fuel injector that comprises a tube assembly having a longitudinal axis extending between a first end and a second end; a seat secured at the second end of the tube assembly and defining an opening; a stem assembly movable with respect to the seat, the stem assembly moving along the axis between a first position wherein the stem assembly contiguously engages the seat such that fuel flow through the opening is prevented and a second position wherein the stem assembly is spaced from the seat such that fuel flow through the opening is permitted; a first set of piezoelectric elements connected to the stem assembly, the first set of piezoelectric elements electromechanically extending and contracting along the axis in response to a first electric field; and a second set of piezoelectric elements connected to the first set of piezoelectric elements, the second set of piezoelectric elements electromechanically extending and contracting along the axis in response to a second electric field.
The present invention also provides a method of actuating a fuel injector. The fuel injector includes a tube assembly having a longitudinal axis extending between a first end and a second end, a seat secured at the second end of the tube assembly and defining an opening, a stem assembly movable with respect to the seat, the stem assembly moving along the axis between a first position wherein the stem assembly contiguously engages the seat such that fuel flow through the opening is prevented and a second position wherein the stem assembly is spaced from the seat such that fuel flow through the opening is permitted, a first set of piezoelectric elements connected to the stem assembly, and a second set of piezoelectric elements connected to the first set of piezoelectric elements. The method comprises applying a first electric field to the first set of piezoelectric elements, the first set of piezoelectric elements electromechanically extending and contracting along the axis in response to the first electric field; and applying a second electric field to the second set of piezoelectric elements, the second set of piezoelectric elements electromechanically extending and contracting along the axis in response to the second electric field.